Exclusive $\rho^0$ Meson Photoproduction with a Leading Neutron at HERA

Abstract

A first measurement is presented of exclusive photoproduction of ρ 0 mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of √ s = 319 GeV and correspond to an integrated luminosity of 1.16 pb −1 . The ρ 0 mesons with transverse momenta p T < 1 GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, x L > 0.35, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality Q 2 < 2 GeV 2 , the total energy of the photon-proton system 20 < W γ p < 100 GeV and the polar angle of the leading neutron θ n < 0.75 mrad. The cross section of the reaction γ p → ρ 0 nπ + is mea- † Deceased a e-mail: sschmitt@mail. Sciences, sured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a ρ 0 meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, σ el (γ π + → ρ 0 π + ), is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction γ p → ρ 0 nπ + .